Microelectromechanical systems (MEMS), which are made up of several micromachined electrical-mechanical structures, have a size typically on a millimeter scale or smaller. These micromachined structures are used to produce MEMS devices that are used in a wide variety of applications including, for example, sensing, electrical and optical switching, and micromachinery (such as robotics and motors). MEMS devices utilize both the mechanical and electrical attributes of a material to achieve desired results. Because of their small size, MEMS devices may be fabricated using semiconductor processing methods and other microfabrication techniques, such as thin-film processing and photolithography.
MEMS technology allows movable microelectromechanical parts to be integrated with micro-optical structures to create MEWS optical devices. These MEMS optical devices may be mass produced at low cost using batch fabrication processes. Moreover, because of their small size and low mass, highly efficient optical devices may be produced with optical MEMS technology. By way of example, applications of optical MEMS technology include optical switches, optical data storage, optical scanners and fiber optic sensors.
One example of a MEMS optical device is a planar all-optical switch. An all-optical switch is capable of switching optical signals between input and output channels without the need for opto-electrical signal conversion. In one embodiment, the all-optical switch includes a MEMS structure having a mirrored surfaced mounted on a movable structure and an actuator for providing force to move the mirrored surface. Using the actuator the mirrored surface can be moved to selectively intercept the path of an optical signal. By selectively intercepting the optical signal, it may be directed to a desired output optical fiber. Such optical switches typically move the mirrored surface about a single axis and utilize only two switch positions, to either intercept or not intercept the optical signal.
In another type of all optical switch, two axes actuators are used. Two-axes actuators are capable of rotating the mirrored surface around two substantially orthogonal axes such that the mirror is capable of being positioned in several different positions. Conventional two-axes optical devices use actuation that includes pads located under the mirrored surface for developing an electrostatic force. In order to position the mirrored surface, the pads are charged such that the mirrored surface is attracted to the charged pads. By selectively positioning the mirror, an optical signal may be directed to one of the many locations.
One problem with this pad-type actuator, however, is that an electric force is generated across a relatively large gap between the pad and the backside of the mirror. This gap exists because the pads are located under the mirrored surface and must allow room for mirror displacement. This large gap effectively decreases the amount of force available to move the mirror, resulting in an increased force requirement, necessitating an increase in system voltage and power to drive the actuator pads. Moreover, with pad-type actuators snap-down can occur due to electrostatic instability occurring because the force from the pad can increase faster that the restoring force from the spring.
Another problem with the above discussed pad-type actuator is that there is no independent control of movement around the two axes in which the mirrored surface rotates. Instead, movement around the two axes is coupled as movement of the mirror along one axis changes the gap distance for the pads under different portions of the mirror. This coupled movement severely limits the angle of scan that can be achieved by the mirror. An additional problem with the coupled movement is that disturbances are harder to correct because damping must occur on both axes. Damping must be applied to the non-disturbed axis as well, because movement around the axes are not independent of each other. Therefore, there exist a need for a manufacturable improved two-axes actuator.